1. Technical Field
This invention is generally related to the technical field of devices for measuring the internal temperature of masses, and more specifically related to the technical field of devices for showing if the internal temperature of a mass has reached a certain value.
2. Prior Art
There are many situations or applications where it is desirable to know or show when the internal temperature of a mass has reached some critical value and visually show this change externally. For example when cooking foods such as turkey, chicken, and roasts it would be useful to show when the internal temperature or doneness has been achieved. This is also true when heating foods in a microwave oven as the heating can be non-uniform. An indicator showing that the entire food item, or at least some internal portion of the food item, has reached some critical temperature throughout the thickness of the food item would be advantageous. Similarly, when shipping perishable foods, vaccines or medications, it is important to know if the internal contents have exceeded some peak temperature.
A number of devices have been used to measure internal temperatures. Volk Enterprises, Inc. uses a spring that is compressed and held in the compressed state by a sharp melting polymer or other material that will release the spring and allow an indicator to become visible when heated to the melt temperature. The spring may not be able to be used in microwave heating applications as the induced eddy currents from the microwave radiation may heat the metal spring and may cause a premature release. Moreover, as the spring subjects the restraining material to some stress, the restraining material may become subject to viscoelasticity near the melt temperature so that the release of the spring may become heating rate dependent. Additionally, it is not a closed system and is subject to contamination.
U.S. Pat. No. 7,387,438 describes a peak temperature indicator that uses a sharp melting column of a material, such as a polymer, wax, metal, or other material, that restrains a float. When the entire column becomes a liquid, the float will rise to the top of a protruding transparent enclosed system. While this concept is simple and easy to use, it does have some deficiencies. The polymers tend to be relatively insensitive to microwave heating; hence the melting of the polymer is dependent on heat transfer from the surrounding food. In addition the polymers expand approximately 8% when heated from its solid crystalline state into its liquid state.
A simpler and less expensive device that avoids these problems is disclosed in the present specification.